1. Field of the Invention
This invention relates to a photosensor to be used in the image inputting section of, for example, facsimile, digital computer or scanner, etc. and an image reading method by use thereof.
2. Related Background Art
In the prior art, for image reading in, for example, a signal sending machine or copying machine for image reading photosensor facscimile, a photosensor for reading a manuscript as shown in FIG. 1 has been used, and reading practiced according to the method as shown in FIG. 2. FIG. 2 illustrates schematically an example by use of a photosensor for reading, in which 1 denotes a photosensor, and a converging type light transmission body 2 such as Selfok lens (trade name: Nippon Itagarasu Co.) is provided below the photosensor, and light emission diode (LED) arrays 3 are provided on both sides thereof, and 4 is a manuscript to be read.
There are various kinds of such photosensors. Those employing non-monocrystallinethin film semiconductors such as amorphous semiconductors or polycrystalline semiconductors, etc. as the photoelectric converting layer are accepted as preferable ones, because non-monocrystalline thin film semiconductors have excellent photoelectric converting functions, and also because enlargement of the thin area can be easily done.
A photosensor using said non-monocrystalline semiconductor as the photoelectric converting layer comprises a photoelectric converting layer comprising a non-monocrystalline layer provided on an electrically insulating substrate. Various methods have been proposed for forming a layer of said non-monocrystalline semiconductor. For example, there is the vacuum vapor deposition method, the ion plating method, the reactive sputtering method, the hot CVD method, the plasma CVD method, the optical CVD method, etc. Among them, the plasma CVD method has been practically applied as the optimum method and is generally known widely.
The constitution of the photosensor of the prior art, as shown in FIG. 1, comprises an electrode 202, a photoconductive member 204 and a transparent electrode 203 successively formed on an insulating substrate 201. The photoconductive member 204 has a single layer structure. For this reason, the photoconductive member 204 had a small dark resistance value. Accordingly, a photosensor of high sensitivity could not be obtained. Although by doping oxygen, etc. into the photoconductive member, the dark resistance of the photoconductive member 204 can be made greater, if too much oxygen, etc. is doped, the light resistance value of the photoconductive member 204 becomes greater, and photosensitivity becomes worse. Therefore, cumbersome operations of controlling oxygen amount, etc. are required. Accordingly, increasing dark resistance of the photoconductive member 204 is of itself of limited value and therefore it has been difficult to obtain a photosensor of high sensitivity.
According to the image reading method shown in FIG. 2 by use of the photosensor shown in FIG. 1, the number of charge generated and collected per photon irradiated is generally smaller than 1, and therefore insufficient current is obtained for driving an external circuit. Accordingly, one amplification circuit must be generally provided for one photosensor forming one picture element. For this reason, the cost of the reading device is extremely high.
Since a photosensor is required to be rather lengthy, an amorphous silicon film has been frequently used for its ease in preparation. However, amorphous silicon film, when irradiated with light for a long time, exhibits degraded characteristics and therefore is reduced in S/N ratio when irradiated with light for a long time.
As described above, in the image reading method of the prior art, since resistance value in the dark and sensitivity during photoirradiation of the photosensor are limited, there is the drawback that the S/N ratio during the reading of image information is too low. Accordingly, a sufficient signal cannot be obtained, and an amplification circuit is required to be mounted for each photosensor, and therefore the cost of the reading device is high. Further, deterioration by light is considerable.